1202 exclusion, 28% tax on unexcluded gain

[tax1111]

Taxpayer sold QSBS in 2020 with substantial gain. He meets requirements and is qualified to exclude 50% of the gain. Based on the tax code, the unexcluded gain should be taxed at 28% rate.
I read the schedule D instruction, and it says if line 15 and line 16 of schedule D is negative, no need to fill out line 18 which shows amount from 28% rate gain worksheet.
But I am confused about the logic behind this:
Why should the 28% tax be ignored if the 1202 gain produces capital loss?
Or Am I missing something?
Will the gain be included in the future year when taxpayer has capital gain?
Thank you!